Spinocerebellar ataxia types 1 (SCA1) and 3 (SCA3) are rare, inherited neurodegenerative disorders that relentlessly progress to total disability and death. SCA1 is the fastest progressing SCA while SCA3 is the most common SCA in US and Europe. Expanded (CAG)n repeats encoding polyglutamines (polyQ) in the respective genes, ATXN1 and ATXN3, cause SCA1 and SCA3. Disease-modifying therapies that target the pathway upstream of the complex pathogenic cascade will offer ultimate treatment. Scientific premise and preclinical animal data strongly support MSK1 inhibitors for SCA1, citalopram for SCA3, and nucleotide-based gene silencing for both SCAs as drugs to be examined in clinical trials in five years. However, the challenge that we face in our current clinical trial readiness for such disease-modifying therapies is that the modest effect size of candidate drugs as measured by the Scale for the Assessment and Rating of Ataxia (SARA; the most robust and well-validated clinical outcome assessment measure) requires large cohorts of study subjects to achieve sufficient statistical power. To accomplish our goal of establishing clinical trial readiness, we propose to launch an international, multi-site effort focusing on premanifest mutation carriers and patients in an early disease stage, who are likely responders to the disease-modifying interventions prior to irreversible brain damage. Based on our studies funded by NIH and the National Ataxia Foundation (NAF), the US ataxia consortium has developed an unprecedented opportunity for tight collaborations with the European Ataxia Study Group to jointly address this challenge and establish clinical trial readiness for SCA1 and SCA3. To achieve our goal, we propose the following specific aims: Aim 1. Establish the world's largest cohorts of premanifest/early SCA1 and SCA3 by combining cohorts, clinical outcome assessment data and biofluid samples (blood, cerebrospinal fluid) from US and Europe Aim 2. Validate MR morphological, biochemical and functional biomarkers in premanifest and early SCA1 and SCA3 Aim 3. Adapt recent developments on statistical design and analysis of small population trials to SCAs.